Characterisation of the surface properties of alpha-lactose monohydrate with inverse gas chromatography, used to detect batch variation

Inverse gas chromatography (IGC) was employed to characterise the surface properties of four batches of alpha-lactose monohydrate supplied as chemically and physically equivalent, but reported to exhibit variable processing performance. These batches were found to be indistinguishable by FT-laser Raman spectroscopy, quantitative X-ray powder diffraction, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and surface area measurement. IGC was undertaken using vapours of polar and non-polar adsorbates or probes introduced individually, in very small quantities, onto the alpha-lactose monohydrate packed column. The net retention volume, V-N, of each probe was determined, allowing the calculation of various surface properties, including the dispersive component of surface free energy (gamma(D)) and specific interaction with polar probes (Delta G(A)(SP)). Whilst the gamma(D) values were comparable, Delta G(A)(SP) values for polar probes demonstrated significant differences, indicating batch variation in the polar surface properties of the four batches of alpha-lactose monohydrate. It is hypothesised that these differences in surface energetics are caused by minor variation in surface crystallinity or purity.